MRAM technology is a non-volatile random access memory technology that may replace present random access memories as the standard memory technology for computing devices. An MRAM cell (also referred to as tunneling magnetoresistive or TMR-device) includes a structure having ferromagnetic layers separated by a non-magnetic layer and being arranged into a magnetic tunnel junction (MTJ). In MRAM cells, digital information is not stored by power but rather is represented as directions of magnetic moment vectors (or magnetizations) in the ferromagnetic layers. More specifically, one of the ferromagnetic layers constitutes a reference layer, the magnetization of which is magnetically fixed or pinned, while the other one of the ferromagnetic layers constitutes a free or storage layer, the magnetization of which is free to be switched between two preferred directions along a preferred magnetization axis (easy axis of magnetization). The free layer magnetization easy axis typically is aligned with the fixed magnetization of the reference layer. In the MTJ's practical use as memory element, one bit of logic information can be assigned to the two different orientations of the free layer magnetization.
Depending upon the two different magnetic states of the free layer (i.e., different directions of magnetization along the easy axis), the MTJ exhibits two different resistance values in response to a voltage applied across the magnetic tunneling junction barrier. Accordingly, the particular resistance of the MTJ reflects the magnetization state of the free layer, such that the electrical resistance is lower when the magnetization of the free layer is parallel to the fixed magnetization of the reference layer than when the free layer magnetization is anti-parallel to the fixed magnetization of the reference layer. Hence, a detection of electric resistance permits “reading” of a particular orientation of the free layer magnetization relative to the fixed magnetization and thus provides the logic information assigned thereto.
In order to switch MRAM cells, magnetic fields which are coupled to the switchable magnetization of the magnetic free layer are applied, which typically are generated by supplying currents to conductive lines, e.g., bit and word lines, that typically cross at right angles with an MRAM cell conventionally being positioned in an intermediate position therebetween and at an intersection thereof (also referred to as “crosspoint-architecture” of MRAM cells).
To be useful in present day electronic devices, MRAM cells need to be arranged in very high-density memory cell arrays. Accordingly, a further down-scaling of individual MRAM cells is seen to be essential to bring MRAM cells into practical use. However, in down-scaling MRAM cells, a number of problems arise that need to be solved. In fact, smaller MRAM cells require higher and higher magnetic switching fields, since, for a given aspect ratio and given free layer thickness, the magnetic switching fields increase roughly like
            1              w              ⁢                  ⁢    or    ⁢                  ⁢          1      w        ,depending on the cell concept, where w is the width of the memory cell. Hence, field selected switching becomes ever more difficult where the width w of the memory cell is decreased and, therefore, large switching currents must be used.
In order to overcome the problem of increased switching currents in smaller MRAM cells, a new concept of MRAM cells featuring domain wall switching has been proposed by the current inventor: see U.S. Pat. No. 6,807,092 B1 to Braun, the disclosure of which is incorporated herein by reference in its entirety.
Reference is now made to FIG. 1, which depicts the basic structure of the new memory cell with domain wall switching. Accordingly, an MRAM cell includes a magnetic tunnel junction (MTJ) 4 stacked in the z-direction, comprised of a magnetic reference layer 1 and a magnetic free layer 2 that are separated by a non-magnetic intermediate layer 3 made of an insulating material that functions as a tunneling barrier. Magnetization 5 of the reference layer 1 is fixed or pinned in a specified direction which for instance is a positive x-direction as depicted in FIG. 1. Otherwise, the magnetic material of the free layer 2 is (or can be) magnetized along a preferred or easy axis of magnetization, while its magnetization is free to be switched between the two preferred directions of the easy axis. The easy axis of the free layer typically is chosen to be aligned with the fixed or pinned reference layer 1 magnetization 5 thus having an x-direction in FIG. 1.
In the new cell concept, free layer 2 is magnetized to have two magnetization components that are oppositely aligned to each other, namely, a first free magnetization 6 positioned on one side (e.g., the left side as depicted in FIG. 1) of the free layer 2 and magnetized in a first direction towards a central portion of the free layer (e.g., the positive x-direction in FIG. 1), and, a second free magnetization 7 positioned on the other side (e.g., right side as depicted in FIG. 1) of the free layer 2 and magnetized in a second direction towards the central portion of the free layer 2 (e.g., the negative x-direction in FIG. 1), where both “sides” are seen to refer to different sides of the free layer 2 along its easy axis of magnetization. Since the first and second free magnetizations 6, 7 are in opposite alignment to each other, a magnetic domain wall 8 (or boundary layer) is created in between them.
In FIG. 1, magnetic reservoirs 9 are disposed below the free layer 2 of the MTJ 4 along opposing edges of the free layer 2, which may be formed from either a soft magnetic material or a hard magnetic material. Both magnetic reservoirs 9 are permanently magnetized in the same direction resulting in magnetizations 10 that are orthogonal to the free layer 2. In FIG. 1, magnetizations 10 point in a positive z-direction.
The magnetizations 10 of the magnetic reservoirs 9 are magnetically coupled to the magnetic free layer 2 to thereby magnetize the magnetic material of the free layer 2 in regions thereof that are adjacent the magnetic reservoirs 9, and, as a result, above-described first and second free magnetizations 6, 7 of the free layer 2 that are oppositely aligned are created.
Magnetic reservoirs 9 are also called “frustrated” magnetic reservoirs (such as is typical in spin glasses) because of the fact that adjacent reservoirs tend to have an opposite alignment of their magnetizations to reduce the overall magnetic energy.
In FIG. 1, a conductive line 11 is disposed between both magnetic reservoirs 9, the magnetic fields of which are magnetically coupled to the first and second free magnetizations 6, 7 of the free layer 2. Magnetic coupling between magnetic fields of the current line 11 and first and second free magnetizations 6, 7 can result in a shift or sweeping out of the magnetically movable domain wall 8 along the free layer 2 easy axis. In other words, depending on the direction of current I propagating through conductive line 11 (positive or negative y-direction), the domain wall 8 is caused to move towards one of the ends of the free layer 2 (positive or negative x-direction). More specifically, in FIG. 1, when a current propagates in the positive y-direction, domain wall 8 is caused to move in the negative x-direction, and vice versa. Inasmuch the domain wall 8 is caused to move in the positive or negative x-direction, a net magnetic moment is created in the free layer 2, resulting in a magnetization that is either in parallel or anti-parallel alignment relative to the fixed magnetization of the reference layer. Reading of the information can be performed using a conventional method of measuring electric resistance of the MTJ 4.
Accordingly, the memory cell can be written to using a single write current made to flow through current line 11 using a transistor switch (not shown) conductively connected to the current line 11. This method is also called “silicon-select.”
Numerical simulations show that the magnetic domain wall between free magnetizations of the free layer is swept out easily from the free layer, when applying small switching currents. The structure thus remains in a stable state (“0” or “1”), depending on whether the boundary layer is swept out to the one or the other side of the free layer along its easy axis. When the current is inverted, switching proceeds by re-creation of the boundary layer and sweeping across the cell towards the other edge. The concept has the advantage of greatly reduced switching currents compared to conventional Stoner-Wohlfahrt switching. For example, in a 40×100×3.75 nm3 sized memory cell, a 2 mA switching current is sufficient to switch the memory cell and to reach a state that is stable after turning off the current. While small switching currents are possible, the need to arrange a comparatively large switch transistor for the memory cell is seen to be detrimental to a further down-scaling of the memory cell.